The printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a versatile thermal processless printing plate material has been noticed which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent, and treated in the same manner as in PS plates.
As the thermal processless printing plate material is cited Thermo Lite described above produced by Agfa Co., Ltd. This plate material is one for short run and its printing durability only provides prints of about 30,000 sheets of coated paper. Since plate inspection after thermal laser exposure is not specifically considered, there is no exposure image visualization property in this plate material.
In contrast, it is printing durability such that a print is made on at least 100,000 sheets of coated paper and image visualization after image recording, as well as the same various printing performances as PS plates, which is requested in the versatile processless printing plate material.
It is expected in the CTP system that procedure of plate inspection will be not carried out in future. However, the plate inspection is still necessary in the present working procedures. Therefore, in the thermal processless printing plate material, image visualization after image recording is necessary, and particularly when a thermal laser is employed for exposure, exposure image visualization is one of the important performances.
In order to form an image for a thermal processless type printing plate material, a thermal laser recording method is used in which the laser emits light having infrared to near-infrared wavelength regions. The processless type printing plate material capable forming an image according to this method is divided into two kinds of printing plate materials described later, an ablation type printing plate material, and a development-on-press type printing plate material which forms a heat fusible image on a printing press.
As the ablation type printing plate materials are cited printing plate materials disclosed in Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These printing plate materials comprise a substrate and a hydrophilic layer and an oleophilic layer provided thereon, either of which is an outer layer. When the outer layer is the hydrophilic layer, the printing plate material is imagewise exposed to ablate the hydrophilic layer to imagewise remove it, whereby the oleophilic layer is unveiled to form image portions at the unveiled portions. However, there are problems that the ablated outer layer scatters and contaminates the exposure device. Therefore, a method is proposed in which a water soluble protective layer is provided on the outer layer, whereby the protective layer prevents the ablated layer from scattering and the ablated layer is removed together with the protective layer on a printing press.
In order to improve printing durability of the ablation type printing plate material, strength of the outer layer need be increased. However, when the printing durability is improved, it lowers sensitivity or resolution degree of images, and therefore, it is difficult to improve printing durability without lowering sensitivity or resolution degree of images.
It is possible to give exposure image visualization to a printing plate material employing an outer layer and a lower layer under the outer layer different in hues from each other. However, in order to realize the visualization, it is necessary to completely ablate and remove the outer layer. This can be realized by suctioning the ablated layer, for example, through a cleaner installed in an exposure device, but the cleaner install results in cost increase.
In the printing plate material described above in which the protective layer is provided, even if hues of the outer layer and the lower layer are different, good exposure image visualization cannot be obtained due to residual ablated layer.
In order to solve the above problem, there has been proposed, for example, in Japanese Patent O.P.I. Publication No. 2002-205466, a printing plate material comprising a hydrophilic overcoat layer, which is capable of being removed on a printing press, containing not less than 20% by weight of a cyanine infrared absorbing dye whose optical density varies due to exposure. This gives good exposure image visualization, and whether a color density of the dye image increases or decreases due to exposure, either exposed potions or unexposed portions exhibit a high color density, however, it is apparent that the high dye content of the layer to be removed on a printing press results in contamination of the printing press due to development on press.
As the development-on-press type printing plate material which forms a heat fusible image on a press, a printing plate material disclosed in JP-2938397 is cited which comprises a hydrophilic layer or a grained aluminum plate and provided thereon, an image forming layer containing thermoplastic particles and a water soluble binder. The Thermo Lite described above produced by Agfa Co., Ltd. is this type of a processless printing plate material.
In order to improve initial printability, a method is considered which increases a content of the water soluble binder in the image forming layer described above. The water soluble binder amount in the image forming layer increased to obtain good initial printability inhibits heat fusibility of the thermoplastic particles. Therefore, this method is difficult to obtain both good initial printability and good image formation.
In order to improve resistance to stain due to scratch (also referred to scratch resistance), it is necessary to prevent lowering of developability on a printing press due to pressure of the thermoplastic particles. Such a method described above in which the water soluble binder content in the image forming layer is increased, or thermoplastic particles of high Tg with high hardness are employed, tends to inhibit image formation, and is difficult to obtain a printing plate material with good scratch resistance and good image formation property.
As a method providing exposure image visualization, a method (as disclosed, for example, in Japanese Patent O.P.I. Publication No. 11-240270) is cited which employs a color fading property due to exposure of an infrared absorbing dye.
As is aforementioned, it has been difficult for prior art to give excellent initial printability, excellent scratch resistance and excellent exposure image visualization to the processless printing plate material.